1. Field of the Invention
The present invention relates to a device for driving an endless belt with belt driving means and rotary bodies arranged side by side in the direction of movement of the belt and driven by the belt. Also, the present invention relates to a color copier, color laser printer or similar color image forming apparatus including belt driving means for moving an intermediate image transfer belt or a sheet conveying belt, and photoconductive drums arranged side by side in the direction of movement of the belt and driven by the belt.
2. Description of the Background Art
Today, an ink jet printing system and an electrophotographic printing system are extensively applied to a low-speed and a medium or high speed color image forming apparatus, respectively. Particularly, high-speed color image forming apparatuses included a tandem color copier.
Japanese Patent Laid-Open Publication Nos. 10-246995 and 63-81373, for example, each disclose a tandem color copier including four photoconductive drums arranged side by side on an endless belt in a direction in which a sheet is conveyed. A particular optical scanning unit is assigned to each photoconductive drum. While each drum is in rotation, the associated scanning unit scans the drum with a light beam in the main scanning direction to thereby form a latent image on the drum. A developing unit also assigned to the drum develops the latent image with cyan, magenta, yellow or black toner for thereby forming a toner image. While the belt conveys a sheet via the four drums, chargers sequentially transfer the resulting toner images of four different colors from the drums to the sheet one above the other, thereby forming a full-color image. After the full-color image has been fixed on the sheet, the sheet or full-color copy is driven out to a copy tray.
The tandem color copier described above forms four toner images on four drums in parallel and transfers the toner images to a sheet by passing the sheet only once. This type of color copier is therefore desirable for high-speed color copying.
In the tandem color copier taught in the above Laid-Open Publication No. 10-246995, the conveying speed of the belt and the peripheral speed of each drum are equal to each other. Each drum is scanned in the main scanning direction at a preselected timing. To bring the toner images of different color into accurate register, press rollers press the drums and belt against each other. In this condition, the belt is caused to turn while causing the drums to rotate. The copier using such press rollers, however, have some problems left unsolved, as will be described hereinafter.
So long as the shaft of each drum is free from eccentricity, the angular velocity of the drum remains constant in accordance with the constant conveying speed of the belt. However, if the shaft is eccentric, then the angular velocity is not constant, as will be described in detail later. The copier therefore brings the toner images out of register or distorts the toner images unless consideration is given not only to eccentricity but also to the variation of the angular velocity. This makes it difficult to provide each drum with a diameter accurate enough to implement resolution as high as 1,200 dpi (dots per inch) or above at low cost. The eccentricity of the shaft causes the angular velocity of the drum to vary and cannot be readily coped with by conventional technologies when high resolution is required.
It is a common practice to detect the eccentricity of a photoconductive drum and control the generation of image data such that a latent image is formed on the drum at the same pitch in the subscanning direction. However, the prior art copier of the type using press rollers renders an image irregular in density due to the eccentricity of the drum.
Further, the drum with eccentricity repeatedly presses the belt in a direction perpendicular to the direction of movement of the belt during one rotation thereof. This phenomenon is allowable if the eccentricity of the drum is small. However, if the eccentricity is great and if the drums differ from each other in the phase of eccentricity, then the belt slackens with the result that the belt and each drum slip on each other.
On the other hand, assume that a motor drives a drive roller via gears or a drive transmission belt. Then, there arise other problems including the variation of transmission speed ascribable to the eccentricity of the gears or rollers (or pulleys) holding the belt, oscillation ascribable to the gears meshing with each other, and the deterioration of the rigidity of the belt. In addition, the torque ripples of the motor, for example, are transferred to the drive roller while being amplified by a speed reduction ratio between the gears or the rollers.
In light of the above, Japanese Patent Laid-Open Publication No. 10-63059, for example, proposes to use a gear train for reducing the output speed of a motor and to mount a large flywheel on the shaft of a photoconductive drum, thereby reducing oscillation ascribable to a transmission system. Although this kind of scheme reduces high-frequency oscillation ascribable to, e.g., the gears, it cannot reduce speed variation ascribable to the eccentricity of the gears or effect accurate control due to the deterioration of the rigidity of the driveline.
Japanese Patent Laid-Open Publication No. 6-271130 discloses a conventional arrangement and a single embodiment of so-called direct drive type in which a motor and a drive roller are constructed integrally with each other without the intermediary of a gear train or similar transmission mechanism. In the conventional arrangement, the motor and drive roller are interconnected by a drive shaft. In the embodiment, an outer rotor pulse motor extends over the entire range of the drive roller, which corresponds to the width of a belt, and causes its outer circumference to rotate. The conventional arrangement is susceptible to extraneous oscillation because, considering the torsional oscillation of the drive shaft which is low in rigidity, the gain of a control system cannot be increased. In the embodiment in which the motor is accommodated in the drive roller, heat is transferred to the belt and lowers image quality. Further, the motor accommodated in the elongate drive roller makes production difficult and increases the cost. Image quality is further lowered by the oscillation of the pulse motor.